The present invention concerns a method for the production of blocked isocyanates from a bis-N-chloramide and an aromatic aldoxime or an aliphatic, cycloaliphatic or aromatic ketoxime.
On account of their high reaction capability and their decomposability by water, isocyanates have been produced, preserved and converted in the form of easily cleavable adducts (blocked or masked isocyanates or isocyanate-precursors). In the coatings industry in particular such isocyanate adducts play an important role. Easily cleavable adducts are obtained from isocyanates and for example phthalimide, imidazole, caprolactam, phenylmethylpyrazolone, diphenylamine, alcohols, phenols, hydrocyanic acid, bisulfite and oximes. (O. Bayer, Angew. Chem. 59 (1947), pp. 257 et seq.) On account of their low cleavage temperature, oxime-blocked isocyanates are of particular interest. Their production follows through the addition of oximes to the isocyanate in an inert milieu, for example in benzene or ether. A drawback to this is that the isocyanate is first synthesized and a water-free solvent must be used with the adduct formation. The oxime-blocked isocyanates obtained in this manner contain traces of free isocyanate, so that toxicity problems occur with their working up. There exists therefore the need for a method which makes possible the direct production of oxime-blocked isocyanates.